The field of the invention pertains to plastic injection molding, and in particular, to plastic injection molding machines.
A typical plastic injection molding machine utilizes a plasticizer screw to melt and move the plastic material, a mold, a gate for the mold and an injector to place the material in the mold. The above components of the machine are connected via a flow channel for the molten material. Hot runner flow channel manifolds are sometimes used to keep the material in a molten state during delivery.
Such a plastic injection molding machine is disclosed in U.S. Pat. No. 4,734,243. This machine includes a plurality of plunger injection devices, one for each mold.
Unfortunately, non-repeatability and the ensuing quality degradation therefrom are sometimes observed with multiple parallel flow channels leading to multiple molds. Part of the problem is the non-Newtonian behavior of polymers which vary in viscosity with flow rate. Variation in tip temperature and flow impedance in the mold cavity are other contributors to the problem.
Therefore, a more effective injection molding machine and method for molding is an advantageous goal. Such a more effective machine and method are disclosed below.
The invention comprises a gear pump disposed in a flow channel of a plastic injection molding machine between the plasticizer and the mold. The gear pump envisioned for use herein is an intermeshing positive displacement interconnected gear pump. Molten plastic in the flow channel passes through the chamber of the gear pump. The gear pump can be either driven internally by the material flowing through the gear pump, or the gear pump can be driven externally by rotating the drive shaft of the pump with a motor.
The flow can be controlled by varying the speed of rotation of the drive shaft. After the mold is filled, the gear pump can be used as a melt pump to continue packing or holding the material in the mold. Reversing the rotation of the gear pump decompresses the material, eliminating gate stringing.
Multiple gear pumps can be used to provide a gear pump in each of several flow channels feeding each of several gates or multiple molds. Multiple gear pumps can be synchronized directly on a single shaft or indirectly by using hydraulic or stepper motors. Alternatively, the multiple gear pumps can be synchronized or non-synchronized on multiple separate shafts. Non-synchronized pumps allow each gate or mold to be filled independently of other gates and molds. Further, it may be advantageous to synchronize the gear pump with the screw of the plasticizer to ensure consistent flow through the machine.